This invention relates to the deposition of thin films on selected areas of a substrate.
Many electronic circuits, such as those incorporating Josephson junction logic and memory devices, include multi-level patterned thin films of resistors, insulators, oxides, and metals formed from a variety of materials such as tin, lead, silver, gold, silicon monoxide, germanium and aluminum. Defining most of these patterns by the commonly used methods (for example, chemically etching or ion milling an evaporated thin film in the presence of a protective photoresist layer) is generally undesirable because of the possible incompatibility of previously defined layers with the etchants or ion beam. This incompatibility has led to a number of stencil or lift-off techniques in which the patterns are defined by evaporating a thin film through a window in a photoresist layer and then removing the undesired material by swelling or dissolving the underlying resist in a suitable stripper. Although these methods alleviate the passivation problems associated with wet or dry etching of thin films, lift-off produces torn edges (burrs) and/or very sharp edges which can cause shorts in junctions, shorts through crossovers or open circuits in patterned films. However, creating a reverse bevel in the photoresist or an undercut stencil is known to eliminate the torn edges.
A stencil technique employing a reverse bevel in the fabrication of Josephson junction devices is described by K. Grebe et al in Journal Vac. Science Technology, Vol. 11, p 458 (1974) and by K. Grebe in U.S. Pat. No. 3,849,136 issued on Nov. 19, 1974. The Grebe et al and Grebe (hereinafter Grebe) technique employs a three layer sandwich comprising two spin coated layers of positive photoresist about 1.5 .mu.m thick separated by a 1 .mu.m layer of evaporated aluminum. When exposed by conventional photoresist exposure equipment, the top layer of photoresist has windows opened in it which correspond to the desired thin film pattern to be evaporated. This patterned resist layer masks the aluminum layer so that the negative image of the final pattern can be etched in the aluminum layer by a suitable acid etchant. During this etch the bottom layer of photoresist protects any previously defined patterns. The bottom layer of photoresist is then grossly overexposed through the aluminum mask so that after development this layer of photoresist undercuts the aluminum by .gtoreq.1 .mu.m. During the overexposure step the top photoresist layer is also exposed and subsequently developed away, leaving the patterned aluminum film as a proximity shadow mask supported 1 .mu.m off the substrate by the bottom photoresist layer. The desired material, up to 1 .mu.m in thickness, is now evaporated through the windows in the aluminum mask with the undercut in the resist preventing the patterned film from being torn during lift-off of the stencil. A soak of the sample in acetone for about 1/2 hour causes the aluminum mask and all the undesired material to lift-off as the underlying layer of photoresist is dissolved.
Because the Grebe method uses an etched aluminum layer as the means of pattern definition, etching of the aluminum is a very critical and limiting factor. Nonuniformity of the etching process over the area of the substrate causes some patterns to undercut the resist layer before others have been completely etched out, resulting in dimensional distortion of comparably sized patterns on the substrate. Other problems with this method are the limited resolution associated with etching very small patterns through a 1 .mu.m aluminum layer, and the difficulty in aligning previous patterns through the 1 .mu.m aluminum layer due to lack of contrast (i.e., low transparency). However, the use of an aluminum layer substantially thinner than 1 .mu.m (0.01-0.10 .mu.m, for example) is, as a practical matter, not feasible because a certain minimum thickness is required to prevent the unsupported aluminum layer overhang from breaking off during processing.
Another disadvantage of the Grebe method is that relatively pronounced undercuts (e.g., &gt;1 .mu.m) can be attained only by a very long exposure (e.g., 10 minutes) of the bottom photoresist. However, long exposure times cause two problems. First, the bottom photoresist swells and distorts the stencil causing a further loss of resolution. Second, a fine residue remains in the vicinity of the edges of the deposited thin film after removal of the stencil and is not as easily removed by immersion in a developer as claimed by Grebe.